There are two kinds of rare earth magnets: sintered magnets and bonded magnets. Recently the bonded magnets are developed quickly. According to the different molding techniques, the bonded magnets are classified as compressing magnets, injection magnets, extrusion magnets and calender magnets. The flexible magnets manufactured by using calender technique are easy to machining, and production cost is low. As a result, there is a great potential for the calender magnets in the applications. Up to data ferrite is the unique magnetic material which has an calender anisotropy and it has been used with a great quantity to produce calender magnets. Although ferrite possesses calender anisotropy, unfortunately, it is ferromagnetic. Its magnetization is low. The maximum energy product of the calender magnets made by ferrite is limited of 5.6-13.6 kJ/m3 (0.7-1.7 MGOe). It is hard to meet the demand to the magnets with small-scale and high performance.
Among the rare earth hard magnetic materials, the NdFeB magnetic powders manufactured by rapid quenching technique are extensively used to produce bonded magnets. However, the NdFeB magnetic powders have some problems when using to make calender magnets. Since their particle size is not small enough, when the NdFeB magnetic powders are used to make calender magnets, the resulted calender magnets have poor flexibility and rough surface, and thus are not satisfied for an ideal machinability. In addition, the NdFeB magnetic powder manufactured by rapid quenching technique is isotropic. Since the NdFeB magnetic powder doesn't present calender anisotropy, the performance of calender magnets based on the magnetic powder is limited. As to the anisotropic rare earth hard magnetic materials, such as Sm—Co magnetic powder or anisotropic NdFeB magnetic powder prepared by HDDR (Hydrogenation, Disproportionation, Desorption, Recombination) process, the term of anisotropy here is only denoted as magnetocrystalline anisotropy. The powders can be oriented when applying a magnetic field. The powders can be used to produce anisotropic compressing or injection magnets with a magnetic field. However, the powders don't present a calender anisotropy. As an example of Sm—Co powder, because it doesn't have calendar anisotropy, the calendar magnets based on Sm—Co powder are isotropic, and its maximum energy product is low, thus having no significance for commercial applications.
Around 1990, J. M. D. Coey et al. disclosed a material having a molecular formula of Sm2Fe17Nδ (J. M. D. Coey et al., “Rare Earth based magnetic materials, production process and use”. European patent Application number: 91303442.7); Iriyama Kyohiko et al. disclosed a similar material based on rare earth, iron, nitrogen and hydrogen (CN 89101552.3); Yingchang Yang et al. made a neutron diffraction study to determine the crystallographic structure of the compounds R2Fe17Nx, indicating the nitride compounds are crystallized in the Th2Zn17-type structure, and nitrogen atoms occupy a specified interstitial site (Yingchang Yang et al. (1991) Neutron diffraction study of ternary nitrides of R2Fe17Nx, Journal of Applied Physics, 70(10): 6018). Due to the interstitial atom effect, the above nitride compound presents a high Curie temperature Tc, a large saturation magnetization Ms, and a strong magnetocrystalline anisotropic field Ha. As a result it is considered as a good candidate for developing a hard magnetic material with strong coercive force iHc, large remanence Br and high maximum energy product (BH)max. How to develop the rare earth-iron-nitrogen based hard magnetic material, people made a great effort. The techniques of smelting, rapid quenching, mechanical alloying, reduction-diffusion, strip-casting, HDDR etc have been applied to produce the different kinds of magnetic powder based on rare earth-iron-nitrogen. However, it is never related to an anisotropic magnetic powder technique for producing anisotropic flexible calender magnets. In addition, it was found that the anisotropic Sm2Fe17Nδ-type magnetic powder presents a large coercive force when the grain size of the powder is only in the range of few μm. However, the magnetic powder with grain size in few μm is easy to oxidize in the atmosphere at room temperature. Accordingly its magnetic properties decline with time because of the oxidation. In particular during the summer season with high humidity the situation is worse. For example of Sm2Fe17N3 powder with a size in 1-3 μm, the intrinsic coercive force at room temperature is measured at the beginning as 11.5 kOe. After 10 weeks, it decreases to 7.0 kOe. Although the remanence doesn't change too much, as the coercive force reduces seriously, the maximum energy product decreases too. The problem of stability is a big trouble in the practical use.
In order to resolve the problem of stability, it was suggested to prepare powder with a large grain size which contains many small crystal by using the techniques of rapid quenching, machinery alloying or HDDR etc. As indicated by ZL 99800830.3, the strip-casting method is used to produce master alloy, then followed by a treatment of hydrogenation, disproportionation, desorption, recombination and nitrogenation. The magnetic powder is obtained. The crystal size is less than 1 μm, its average diameter is in a range of 0.1-1.0 μm. That allows to create a large coercive force. However, the average diameter of the powder is 10-300 μm, that improves the stability of the powder. However, the powder is isotropic. That means the stability of the powder is improved at the cost of reducing its magnetic performance, and said powder is merely used to produce isotropic molding magnets.
In brief up to date the magnetic powder based on rare earth-iron-nitrogen obtained by using different manufacture methods or adding various elements is either isotropic, or anisotropic only when applying a magnetic field. All of them don't present a calender anisotropy. Obviously the above powder cannot be used to produce anisotropic calender magnets. In order to satisfy the requirement of reducing the scale of device, it is need to develop various anisotropic rare earth bonded magnets with high performance, especially anisotropic flexible calender magnetic. At the same time in order to meet the requirement of practical use it is necessary to resolve the problem of stability of the anisotropic magnetic powders. However, there is no any rare earth hard magnetic material can satisfy the above requirements to date.